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Weathering and Soil Formation
Ayers Rock… Another View
Coach Holmes indicator for,
“How far out in the boonies are we?”
Distance from civilization.
Approximately how far away from Ayers
Rock is the closest McDonalds restaurant?
465 km (289 miles). The closest
McDonalds is in Alice Springs. 22
km to Yulara then another 443 km
to Alice Springs
Essential Question:
 NCSCOS: EEn.2.1.3
 Explain how natural actions such as weathering,
erosion(wind, water and gravity), and soil formation
affect the Earth’s surface.
 E.Q. - What are the mechanisms of weathering
Objectives and Goals
 Recall that soil is the result of weathering of rocks and
includes weathered particles: sand, silt and clay.
 Explain differences in chemical and physical
weathering how weathering rates are affected by a
variety of factors including climate, topography and
rock composition.
 In your journals:
 Give your definition of weathering.
Part I
 The breaking down of rocks and
other materials on the Earth’s
surface is called weathering. A slow,
continuous process, it affects all
substances exposed to the
Types of Weathering
•Rocks on the Earth’s
surface are broken down
by two types of
•mechanical and chemical.
Mechanical Weathering
 When the forces of weathering break rocks
into smaller pieces but do not change the
chemical makeup of the rocks, the process
is called mechanical weathering. During
mechanical weathering, rocks are broken
into different shapes and smaller pieces.
At the beginning the edges are jagged, as
weathering continues, they become round.
Causes of Mechanical Weathering
 Must involve motion or movement
 Temperature / Exfoliation
 Frost action / Ice wedging
 Organic activity / Root Pry
 Gravity / Rolling down hill
 Abrasion / Sand blasting
 Glacier / Till
 Rocks can be broken apart by
changes in temperature. As rocks
are heat up in the sun during the
day, the outside of the rock
expands. The inside of the rocks
remain cool and do not expand.
When the air temperature drops at
night, the outside of the rock cools
and contracts. This continuing
cycle causes particles to break off.
This is called exfoliation.
Rock Exfoliates
Study the figure and answer the questions
below in your notebook.
a Are the changes in temperature gradual or rapid?
b In which regions do these fluctuating temperatures often occur?
c Where are the points of weakness in the rock?
Frost Action
 Unlike most liquids, water expands when it freezes.
The repeated freezing and melting of water, called
frost action, is another cause of mechanical
weathering. When water freezes in cracks in the
rocks, it expands, making the crack larger.In time, this
causes the rock to break into pieces.
Ice Wedging / Frost Action
Study figure and answer the questions
below in your notebook
 a Where is the rainwater going?
 b What happens when temperatures fall below 0°C?
 c Why has the crack widened?
 d What effect will this have on the rock?
 e Which process does this flow chart illustrate?
Organic Activity
 Plants and animals can cause
mechanical weathering. The roots of
plants sometimes loosens rock
material. A plant growing in a crack
can make the crack larger as the root
spread out. This is known as root-pry.
It is organic since this activity is caused
by living things.
Study the figure and answer the questions
below in your notebook.
a What happens to the crack as the roots get bigger?
b What kind of weathering is this?
c What kind of weathering does this process cause?
d The decaying tree will provide…
 Gravity is another agent of mechanical
weathering. Sometimes gravity pulls
loosened rocks down mountain cliffs in
a landslide. A landslide is a large
movement of loose rocks and soil, know
as MASS MOVEMENT. As the rocks fall,
they collide with one another and break
into smaller pieces. Falling rocks usually
occur in areas where a road has been cut
through, leaving cliffs on both sides.
 Wind-blown sand causes mechanical
weathering . Abrasion is the wearing
away of rocks by solid particles
carried by wind, water or other forces.
In desert regions or at the beach, the
wind easily picks up and moves sand.
The sharp edges of the sand particles
scrape off pieces of exposed rocks.
Running water also carries loose rocks
which scrape against each other and
Sandblasting Art
Chemical Weathering
When the chemical makeup of the rocks is
changed it is called chemical weathering.
During chemical weathering, changes
occur in the mineral composition of rocks.
Minerals can be added, removed or broken
down (decomposed). Many substances
react chemically with rocks to break them
Types of Chemical Weathering
 The chemical composition must be
 Water /dissolving
 Oxidation / Rusting
 Carbonation / Acid Rain weak
 Sulfuric acid / Acid Rain strong
 Plant acids / Mosses and lichens
Most chemical weathering is caused by
water and carbon dioxide. Water can
dissolve most of the mineral that hold
rocks together. Rocks that dissolve in
water are said to be soluble. Water can
form acids when it mixes with certain
gases in the atmosphere to speed up the
decomposition of rocks. Water can also
combine with a mineral to form a new
Our State (Magazine)
 Linville Caverns Video
 Chemical weathering is also caused by
oxidation. Oxidation is the process in
which oxygen chemically combines with
another substance. The result of oxidation
is the formation of an entirely different
substance. Iron in rocks combines with
oxygen in the air to form iron oxide, or
 When carbon dioxide dissolves in water, a
weak acid called carbonic acid is formed.
When carbonic acid reacts chemically with
other substance, the process of carbonation
occurs. In nature, carbonic acid is formed
when carbon dioxide in the air dissolves in
rain. This acid rain falls to the ground and
sinks into the soil. It decomposes feldspar
and limestone.
Study the figure and answer the questions
below in your notebook.
a What kind of weathering is this?
b How does this kind of weathering affect the rock?
c Which climates speed up this kind of weathering?
Sulfuric Acid
 The air in certain areas is polluted with
sulfur oxides. Sulfur oxides are a byproduct
of the burning of coal as a source of energy.
These compounds dissolve in rainwater to
form sulfuric acid. Rain that contains
sulfuric acid is one type of acid rain. It is
much stronger than carbonic acid. Sulfuric
acid corrodes rocks, metals and other
materials quickly.
Plant Acids
 Plants produce weak acids that dissolve
certain minerals in rocks. Mosses and
lichens produce weak acids that dissolve
some of the minerals in the rocks they grow
on in areas of PERMAFROST / Polar
Climates. Gradually the rocks break into
smaller pieces. They are important in the
formation of soil.
Rate of Weathering
 The composition of the rock
 The amount of time that the rock is
exposed on the Earth’s surface
 The amount of exposed surface on a
 Climate
 Relief / Topography (Height)
Rate of Getting Wet
Check your understanding of weathering rates.
With a partner list five different
ways that your clothes become
saturated with rain more and/or
faster. Then compare that list with
your notes on the rates of
Rate of Weathering Graph
Composition of Rocks
 Two different types of rocks in the same
climate can weather differently, depending
on the minerals that make up each rock
type. If a rock resist weathering, the rock
is called a STABLE ROCK. The stability of a
rock can vary depending on the climate in
which the rock is found. Limestone is
stable in a dry climate but not in a wet
Amount of Time of Exposure
 The amount of time that rock is
exposed on the Earth’s surface also
affects its rate of weathering. A very
old rock that has not been exposed
to the forces of weathering can
remain almost unchanged. If a
newly formed rock is deposited on
the Earth’s surface it will begin to
weather right away.
Which Is Older????
The Amount of Exposed Surface
 The amount of exposed surface area on a
rock also affects its rate of weathering.
As rocks are broken down into many
small pieces, more rock surfaces are
exposed and more weathering takes
place. In rocks that contain many joints
or cracks, various chemicals easily come
into contact with the rock surfaces and
break them down.
Which Dome is more exposed???
Chimney Tops… G.S.M.N.P.
 Cold and/or dry climates favor
physical weathering. Arctic/Polar
 Warm and wet climates favor chemical
weathering. Tropical
 Frost action works best in areas
where the temperature fluctuates
wildly. Temperate Climates
Part 2
Part II Erosion and Deposition
 Essential Question:
 Compare erosion by water, wind, ice, and gravity
and the effect on various landforms
What Caused This?
What is Erosion?
Erosion – the moving of rock material from
one place to a new location
 For erosion to occur three processes must
take place: detachment of particles, lifting
them, and transporting them
 Many agents of erosion - flowing water,
moving ice, waves, gravity, or wind
 Sand, silt and clay consists of small pieces
of rock that have been weathered from a
parent rock, eroded, and deposited
somewhere else
What Is Wind Erosion?
 Wind - responsible for wearing away rocks and
creating great deserts like the Sahara Desert and
 Most effective in moving loose material
 Two main effects: (1) Wind causes small particles to be
lifted and moved away. (2) Suspended particles may
impact on solid objects causing weathering by abrasion
 Occurs in areas where there is not enough rainfall to
support vegetation
What Is Water Erosion?
 Water - most influential force in erosion
 Ability to move materials from one location to another
over long distances
 The faster water moves in streams the larger objects it
can pick up and transport. Known as STREAM LOAD
 Responsible for wearing away of rocks in rivers, lakes,
and the oceans
What Is Wave Erosion?
 Waves - relentless pounding
 Erodes the softer, weaker parts of the rock first, leaving
harder, more resistant rock behind
 Can take over 100 years to erode a rock to sand
 Energy of waves along with the chemical content of the
water erodes the rock off the coastline
What Is Gravitational Erosion?
 Mass movement
- downward movement of rock
and sediments, mainly due to the force of gravity.
 Moves material from higher elevations to lower
elevations where streams and glaciers can pick up the
material and move it to lower elevations
 Process is occurring continuously on all slopes, some act
very slowly while others occur very suddenly until
equilibrium is reached
What Is Glacial Erosion?
 Ice
- moves and carries rocks, grinding the rocks
beneath the glacier
 Weathering Abrasion cuts into the rock under the
glacier, smoothing and polishing the rock surface
 With the same glacial process material is transported to
new locations.
 HOMES ???
What is Deposition?
 Deposition - laying down of sediment that
has been eroded/transported by a medium
such as wind, water, waves, gravity or ice
 Process of erosion stops when the moving
particles fall out of the transporting medium
and settle on a surface. This settling of
eroded material into a new location is called
 Obstacles, whether natural or man-made,
will often decide where the deposition
occurs and the nature of the feature formed.
What’s the Difference?
 WEATHERING - think weather wearing rock down
 EROSION - think of a road and traveling
 DEPOSITION – think of depositing money in a bank
Part III Soils
 “What is Soil ?”
What is Soil?
 Soil - Soil is a natural body comprised of
solids (weathered minerals, rock and
decaying organic matter called humus),
liquid (water), and gases (air) that occurs on
the land surface, occupies space, and is
characterized by horizons, or layers, that are
distinguishable from the initial material as a
result of additions, losses, transfers, and
transformations of energy and matter or the
ability to support rooted plants in a natural
What is Dirt???
 Often referred to mistakenly as soil.
Dirt is actually unclean matter,
especially when it is in contact with a
person's clothes, skin or possessions
when they are said to become dirty.
Common types of dirt include mud,
dust and, yes, soil itself. Any unclean
substance, such as mud, dust,
excrement, etc.; filth
What’s the difference between soil
and dirt?
 Dirt is what you find under your fingernails.
Soil is what you find under your feet.
 Think of soil as a thin living skin that
covers the land. It goes down into the
ground just a short way. Even the most
fertile topsoil is only a foot or so deep. Soil
is more than rock particles. It includes all
the living things and the materials they
make or change.
Soil Formation
 The weathering of rocks on the Earth’s
surface results in the formation of soil.
 Soil is formed when rocks are
continuously broken down by
 As rocks weather, they break into
smaller pieces.
 These pieces are broken down into even
smaller pieces to form soil.
Importance of Soil
 The formation of soil is extremely
important to most living organisms.
 Plants depend on soil as source of food.
 Soil supplies plants with minerals and
water needed for growth.
 Animals depend indirectly on soil since
they eat plants and other animals that
eat plants.
Residual Soil; Lives at Home
 Sometimes soil remains on top of
its parent rock, or the rock from
which it was formed. This is called
residual soil. Residual soil has a
composition similar to that of the
parent rock it covers.
Transported Soil; Erosion
 Some soil is removed from the
parent rock by water, wind, glaciers
and waves. Soil that is moved away
from its place of origin is called
transported soil. Transported soil
can be very different in composition
from the rock it covers.
 The layer of rock beneath the soil is
called bedrock. Also Known as
Parent Rock
 Certain bacteria in the soil cause the
decay of dead plants and animals.
 This decaying material is called humus.
 Humus is a dark-colored material that
is important for the growth of plants.
 Some of the chemicals produced during
the process of decay speed up the
breakdown of rocks into soil. Acids
Living Things
 Living things such as moles,
earthworms, ants and beetles help to
break apart large pieces of soil as they
burrow through the ground. The
burrows allow water to move rapidly
through the soil. The water speeds up
the weathering of the underlying rock.
Soil Composition
 Pieces of weathered rock and organic
material, or humus, are the two main
ingredients of soil. Organic materials is
material that was once living or was
formed by the activity of living
organisms. Rock particles form more
than 80% of soil. Air and water are also
present in soil.
Minerals in Soil
 Clay and quartz are the most abundant
minerals in soil. Because they are
stable, they exist in the greatest
quantities. Potassium, phosphorus and
the nitrogen compounds called nitrates
are important chemicals in soil. They are
vital to plant growth.
Pore Spaces
 Air and water fill the spaces between soil
particles. These are called pore spaces.
Plants and animals use the water and air
in these spaces, as well as the minerals
dissolved in water. Pore spaces provide
needed oxygen for healthy plant root
Different Compositions of Soil
 The composition of soil varies from place to
place. The type of rock broken down by
weathering determines the kinds of minerals
in the soil. The type of weathering also affects
the composition of soil.
 Mechanical weathering produces soil with a
composition similar to the rock being
 Chemical weathering produces soil with a
different composition.
Soil Texture
 The type of weathering also affects soil
texture. Texture refers to the size of the
individual soil particles. Soil particles
vary from very small to large. Both
mechanical and chemical weathering
first breaks rocks into gravel (2-64mm)
and then in sand (less than 2mm) and
then into silt and finally clay.
Soil Horizons
 As soil forms, it develops separate
soil layers called horizons. Each
soil horizon is different. A cross
section of the soil horizons is called
soil profile. A soil profile shows the
different layers of soil.
Evolution of Soil Horizons
Topsoil; Root Zone
Parent Rock
New Horizon Classification
Mature Soil
 Soil that has developed three layers is
called mature soil. It takes thousands
of years and the proper conditions for
soil to develop three layers. The
uppermost layer of mature soil is
called the A horizon. The A horizon is
a dark-colored soil layer in which
much activity by living organisms
takes place. Bacteria, earthworms
and beetles help the decay.
O (Organic) Horizon
 This is the organic debris that has
recently accumulated on the surface
of the soil. It has begun to
A Horizon
 The soil in the A horizon is called
topsoil. Topsoil consists mostly of
humus and other organic materials.
Humus supplies minerals essential
for plant growth. Humus is spongy
and stores water. It also contains
pore space for air and water. Topsoil
is the most fertile part of the soil.
E (Eluviation) Horizon
 This eluviation (leaching) layer is
light in color; this layer is beneath
the A Horizon and above the B
Horizon. It is made up mostly of sand
and silt, having lost most of its
minerals and clay as water drips
through the soil (in the process of
Virgin Soil
 Soil that has little or no humus/organic mater
incorporated. “B” horizon and below
 Soil that is as yet undeveloped
 Soil that has not been cultivated before
 Virgin soil, when compact and of considerable depth,
is a good material to build on, providing the structure
is not an extremely lofty or heavy one.
B Horizon
 Water that soaks into the ground
washes some minerals from the A
horizon into the second layer of soil,
or the B horizon. This process is
called leaching. The B horizon is just
below the A horizon. The B horizon is
also made of clay and some humus.
The soil in the B horizon is called
subsoil. Subsoil is formed very slowly.
C Horizon
 The third layer of soil is called the
C horizon. The C horizon consists
of partly weathered parent rock.
The C horizon extends down to the
top of the un-weathered parent
rock. The composition of the soil
in the C horizon is similar to that of
the parent rock.
What are the differences between
eluviation and illuviation?
Eluviation is the leaching down
or movement of the particles (such
as minerals and organic matter)
into lower soil horizons. While,
Illuviation is the accumulation
of those particles in the lower soil
R(rock) Horizon
 Solid un-weathered bedrock. Referred
to Parent Rock. It is either…
 Permeable – allowing most substance to
pass or flow in and out
 Semipermeable - allowing some
substances to pass
 Impermeable or Nonpermeable –
materials cannot pass through
Immature Soil
In some places, the upper layers of soil
are removed and the rocks below the soil
are exposed. The weathering process
then forms new soil from the exposed
rocks. This recently formed soil is
immature because there has not been
enough time for all three soil layers to
form. The soil in the northern regions
where glacial erosion has taken place, is
immature soil.
Formation of Soil
 There are several factors that
determine whether three layers of soil
will form.
 Time
 Climate
 Type of rock / Parent Rock
 Surface features of the region
 Time is one of the most important
factors in soil formation. The longer a
rock is exposed to the forces of
weathering, the more it is broken down.
Mature soil is formed if all three layers
have had time to develop.
 Climate is another important factor in
the formation of soil. In areas with
heavy rainfall and warm temperatures,
weathering takes place more rapidly.
Heavy rainfall may wash much of the
topsoil away. Since Organisms are more
plentiful these areas, the soil is quickly
replaced. They speed up the chemical
and mechanical weathering of rocks.
Type of Rock
 The type of rock in an area also affects
soil formation. Some rocks do not
weather as rapidly as other do. Rocks
that do not break down easily do not
form soil rapidly. In some climates it
takes along time for granite to break
down. So soil formation from granite is
slow. But sandstone breaks easily and
forms soil quickly.
Surface Features of Region
 The surface features of the region
also determine the speed at which
soil is formed. On very steep
slopes, rainwater running off the
land erodes the soil and exposes
rock to weathering.
Four Soil Formation Processes
 These processes of soil genesis, operating under the
influence of environmental factors, give us a logical
framework for understanding the relationships
between particular soils and the landscapes and
ecosystems in which they function. This approach is
known as internal process modeling. In analyzing
these relationships for a given site, ask yourself:
Ask yourself…
 What are the materials being added to this soil?
 What transformations and translocations are
taking place in this profile?
 What materials are being removed?
 How do climate, organisms, topography, and
parent material affect these processes over time?